The present invention relates generally to refrigeration circuits and, more particularly, to an apparatus to effect defrost of an outdoor heat exchanger incorporated in an air conditioning unit such as a heat pump.
In a conventional refrigeration circuit, a refrigerant compressor, condenser, expansion means, and evaporator are connected to form a refrigerant flow circuit. In operation, the compressor raises the temperature and pressure of gaseous refrigerant, and the gaseous refrigerant is directed to the condenser where it gives off heat energy and is condensed to a liquid. The liquid refrigerant is then directed through an expansion means which reduces its pressure enabling it to be capable of changing from a liquid to a gas absorbing heat energy. The refrigerant is then directed into an evaporator where a change of state from a liquid to gas occurs as heat energy is removed from the media flowing in heat transfer relation with the evaporator. The gaseous refrigerant from the evaporator is then directed back to the compressor.
When a refrigeration circuit is operated under certain ambient conditions, moisture may be deposited on the coil surfaces of the evaporator, and the evaporator coil may develop ice thereon. When ice or frost develops on the coils of the evaporator, the efficiency of the heat exchanger and of the entire system decreases.
Formation of ice on the evaporator coils is particularly a problem with heat pumps since the coil of the outdoor heat exchanger operates as the evaporator when the unit is used to heat a defined enclosed area. Therefore, it is desirable in the operation of a refrigeration circuit, such as a heat pump, to maintain the evaporator coils free of frost and ice. In addition, it is desirable when defrosting the outdoor evaporator coils in a defrost mode, to prevent cold air from being discharged into the enclosed area, and to do so without the necessity of supplemental inefficient electric heat elements such as heat strips being necessary and activated during the defrost cycle.
A number of systems are known for defrosting the evaporator coils in a refrigeration cycle. For example, U.S. Pat. No. 4,171,622 to Yamaguchi et al. provides a heat pump which includes an auxiliary heat exchanger that acts as both a defroster and a subcooler. Refrigerant is directed through a separated coil in the outdoor heat exchanger, which coil acts as a subcooler and a defroster. There is no disclosure of bypassing the indoor heat exchanger, nor is there disclosure of directing hot gaseous refrigerant through a coil in an auxiliary heat exchanger that is only utilized during a defrost mode when the indoor heat exchanger is bypassed.
U.S. Pat. No. 4,565,070 to Raymond, discloses a refrigeration system which incorporates a bypass circuit to bypass the indoor heat exchanger during the defrost mode. This system utilizes two outdoor heat exchangers which alternate operation from evaporator to condenser, one at a time, during the defrost cycle so that each outdoor heat exchanger can be defrosted. During the heating and cooling cycle, refrigerant is directed in series through both outdoor heat exchangers.
U.S. Pat. No. 4,197,716 to Nussbaum discloses a refrigeration circuit which includes a defrost cycle for defrosting the evaporator while bypassing the condenser. Such a system utilizes an auxiliary heat exchanger which is connected in series during the cooling cycle so as to act as a supercooler, and which acts as a condenser during the defrost cycle when the main condenser is bypassed.
U.S. Pat. No. 4,774,813 to Yokoyama, U.S. Pat. No. 4,916,913 to Narikiyo, U.S. Pat. No. 2,928,255 to Harnish and U.S. Pat. No. 4,727,727 to Reedy also disclose various types of heat exchangers.
Thus, various refrigeration systems have been provided that include means for defrosting ice or frost that forms on evaporator coils. However, none provide for the efficient operation of the heat pump and defrost of the outdoor heat exchanger coils while not discharging cold air into the enclosed area, without supplemental heat such as heat strips during normal operation, in the manner as provided by the present invention.